net/http: clarify Header.WriteSubset behavior

The current implementation performs a plain map lookup,
but other header methods canonicalize header keys before
using them.

Fixes #34918

Change-Id: Id4120488b8b39ecee97fa7a6ad8a34158687ffcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/201357
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/header.go

@@ -178,6 +178,7 @@ func (h Header) sortedKeyValues(exclude map[string]bool) (kvs []keyValues, hs *h
 
 // WriteSubset writes a header in wire format.
 // If exclude is not nil, keys where exclude[key] == true are not written.
+// Keys are not canonicalized before checking the exclude map.
 func (h Header) WriteSubset(w io.Writer, exclude map[string]bool) error {
 	return h.writeSubset(w, exclude, nil)
 }